1. Field of the Invention
The present invention generally relates to medical methods and devices. More particularly, the present invention relates to methods and regulated or modulated constraining devices used for holding structures together. These constraint devices may be used to hold anatomical structures such as bones together with higher holding forces at the beginning of the healing process when more support is needed and lower holding forces later on as healing progresses and less support is desired. Other embodiments work in an opposite manner and provide increased holding forces over time. By varying the holding forces over time, a single device may be used to accommodate different phases of the healing process and automatically adjust the length or tension in the constraining device without requiring additional surgery. Exemplary internal fixation procedures include repair of fractured bones and cerclage of a spinal segment in patients with segmental back pain or segmental instability such as in degenerative spondylolisthesis.
Current repair of fractured bones may involve the use of an internal fixation device. For example, an exemplary use of an internal fixation device is to facilitate repair of a fractured greater trochanter. In current procedures, a stiff cable may be wrapped around the fractured portions of bone in order to hold them together during healing. The cable typically provides relatively constant holding forces over the course of the healing process. This is not always desirable. In some cases, it is desirable to firmly hold the fractured bones together at the beginning of the healing process. However, over time as the bone heals, a constant fixation force can stress shield the tissue which can hamper healing later on as the tissue remodels to accommodate a lower load. Higher compressive forces may also cause bone necrosis. Additionally, it may be desirable to allow a small amount of micromotion or a small amount of loading in order to promote tissue repair. Thus it would be advantageous to provide a device that can vary the holding forces as the healing process advances. In other cases, it might be advantageous to provide a device that increases the holding force over time. This allows slack or loosening that may develop over time to be taken up as healing progresses.
Internal fixation is also used to treat patients with low back pain resulting from internal disc disruption or discogenic disease. This includes the use of bone cerclage devices. There are a number of different types of cerclage devices. One type uses an inelastic cable to encircle the spinous processes or other vertebral elements thereby restraining motion. Another type of cerclage involves the use of an elastic tether structure coupled to the spinal segment. Some cerclage devices include the use of a spacer implanted between adjacent spinous processes. While these approaches are promising, they all have potential shortcomings. For example, some of these devices are invasive and require removal of surrounding ligaments, and others are designed for static applications and thus limit motion of the affected spinal segment. This may be advantageous at the beginning of healing when immobilization helps improve the likelihood of concomitant fusion but is undesirable later on in the healing process when some motion is desirable. Still other devices allow substantially unrestricted spinal extension but the constraining force provided by the device is either pre-set or related to the extent of spinal segment motion and therefore the devices do not vary the fixation forces during the course of the healing process.
Discogenic disease is also associated with degenerative spondylolisthesis, a spinal condition in which abnormal segmental translation is exacerbated by segmental flexion. Treatment for degenerative spondylolisthesis often involves removal of bone or other tissue causing the nerve impingement (decompression surgery), combined with bone fusion to prevent further instability. Since bone fusion takes time, often the entire segment is also immobilized or stabilized (sometimes referred to as fused) using instrumentation, for example, pedicle screws and stabilization rods. This relatively rigid instrumentation often offloads the implanted fusion material, which needs some loading for strong bone formation. Moreover, the instrumentation remains rigid over time. Therefore, again it would be useful to provide a fixation device that can vary the holding or fixation forces over the course of the healing process.
While the constraint devices discussed above are promising, they often provide consistent fixation until they are removed, if at all. Thus the constraint devices shield the healing tissue from stress. This is often desirable at the early phase of healing, but can hamper tissue repair as the tissue remodels. Additionally, in some cases, it is also desirable to permit a small amount of motion or a small amount of loading, as this facilitates tissue repair. Furthermore, it may be desirable initially to protect the healing tissue from high impact loading through the use of a damping mechanism. Therefore, for the aforementioned reasons there is a need to provide a constraint device which can vary the properties of the device, such as holding force, during the healing or tissue regeneration process. In particular, such a device should be minimally invasive and easily adjustable by a physician during the healing process. It would also be desirable for such a device to automatically vary the device's constraining properties over time. Additionally, the device's constraining properties could vary based on sensing the evolving mechanical or biochemical environment or based on an induced change to the device environment.
2. Description of the Background Art
Patents and published applications of interest include: U.S. Pat. Nos. 3,648,691; 4,643,178; 4,743,260; 4,966,600; 5,011,494; 5,092,866; 5,116,340; 5,180,393; 5,282,863; 5,395,374; 5,415,658; 5,415,661; 5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,562,737; 5,609,634; 5,628,756; 5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133; 5,964,769; 5,989,256; 6,053,921; 6,248,106; 6,312,431; 6,364,883; 6,378,289; 6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091; 6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729; 6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245; 6,761,720; 6,835,205; 7,029,475; 7,163,558; Published U.S. Patent Application Nos. US 2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US 2004/0117017; US 2004/0127989; US 2004/0172132; US 2004/0243239; US 2005/0033435; US 2005/0049708; 2005/0192581; 2005/0216017; US 2006/0069447; US 2006/0136060; US 2006/0240533; US 2007/0213829; US 2007/0233096; 2008/0009866; 2008/0108993; Published PCT Application Nos. WO 01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO 03/045262 A1; WO2004/052246 A1; WO 2004/073532 A1; WO2008/051806; WO2008/051423; WO2008/051801; WO2008/051802; and Published Foreign Application Nos. EP0322334 A1; and FR 2 681 525 A1. The mechanical properties of flexible constraints applied to spinal segments are described in Papp et al. (1997) Spine 22:151-155; Dickman et al. (1997) Spine 22:596-604; and Garner et al. (2002) Eur. Spine J. S186-S191; Al Baz et al. (1995) Spine 20, No. 11, 1241-1244; Heller, (1997) Arch. Orthopedic and Trauma Surgery, 117, No. 1-2:96-99; Leahy et al. (2000) Proc. Inst. Mech. Eng. Part H: J. Eng. Med. 214, No. 5: 489-495; Minns et al., (1997) Spine 22 No. 16:1819-1825; Miyasaka et al. (2000) Spine 25, No. 6: 732-737; Shepherd et al. (2000) Spine 25, No. 3: 319-323; Shepherd (2001) Medical Eng. Phys. 23, No. 2: 135-141; and Voydeville et al (1992) Orthop Traumatol 2:259-264.